Cochlear implant systems (also known as cochlear prostheses) have been developed to help overcome certain types of hearing loss. For example, a cochlear implant system may include a cochlear implant (i.e., a stimulator implanted within a patient) that bypasses hair cells in the patient's cochlea by applying electrical stimulation directly to auditory nerve fibers by way of an array of electrodes implanted in the cochlea. Direct stimulation of the auditory nerve fibers in this way leads to the perception of sound in the patient's brain and at least partial restoration of hearing function. Additionally, for cochlear implant patients who retain at least some amount of residual hearing, certain cochlear implant systems may be employed that leverage and help preserve the residual hearing. Such cochlear implant systems may be known as electro-acoustic stimulation (“EAS”) systems, and, for example, may use a loudspeaker to apply, in conjunction with the electrical stimulation applied by the cochlear implant, acoustic stimulation to functional hair cells in the cochlea.
When a cochlear implant is initially implanted in a patient (e.g., as part of a regular cochlear implant system or an EAS system), as well as during follow-up tests and checkups thereafter, it may be desirable to fit the cochlear implant to the patient. Such “fitting” may include adjustment (e.g., within a sound processor that directs and/or controls the cochlear implant) of a base amplitude or intensity of various stimuli generated by the cochlear implant from the factory settings or default values to values that are most effective and comfortable for the patient. For example, the intensity or amplitude and/or duration of the individual stimulation applications (i.e., continuous bursts of stimulation pulses) provided by the cochlear implant system may be mapped to an appropriate dynamic audio range so that the appropriate “loudness” of sensed audio signals is perceived. Ultimately, by properly fitting a cochlear implant (i.e., fitting the cochlear implant system) to a patient, it may be ensured that loud sounds are perceived by the patient as loud, but not painfully loud, and that soft sounds are perceived by the patient at soft levels, but not such soft levels that the sounds are not perceivable at all.
One aspect of fitting a cochlear implant to a particular patient is determining at least one most comfortable level (“MCL”), also known as a “most comfortable current level” or an “M level.” An MCL refers to a stimulation current level applied by a cochlear implant system at which the patient is most comfortable. MCLs typically vary from patient to patient and from electrode channel to electrode channel in a multichannel cochlear implant.
MCLs are typically determined based on subjective feedback provided by cochlear implant patients. For example, a clinician may present various stimuli to a patient and then analyze subjective feedback provided by the patient as to how the stimuli were perceived. Subjective feedback may take the form of verbal feedback (e.g., in the case of adults), or non-verbal feedback (e.g., in the case of certain children). Unfortunately, relying on subjective feedback in this manner is difficult, particularly for those patients who may have never heard sound before and/or who have never heard electrically-generated “sound.” For young children, the problem is further exacerbated by short attention spans and/or difficulty in understanding instructions and concepts (e.g., high and low pitch, softer and louder, same and different, etc.). Moreover, certain patients such as infants and/or patients with certain disabilities may be unable to provide any subjective feedback.
Consequently, it may be desirable to determine MCLs of certain cochlear implant patients by way of an objective technique, rather than by means of the subjective feedback. To this end, it is known that the MCLs of most patients tend to be highly correlated with (e.g., in many cases, substantially equal to or slightly offset from) stapedius reflex thresholds (“SRTs”) of the patients, which may be determined objectively (i.e., without requiring subjective feedback). An SRT of a particular patient is the threshold loudness level at which the patient's brain triggers a stapedius reflex, a natural hearing protection mechanism in humans whereby the stapedius muscle of the middle ear involuntarily contracts in response to uncomfortably or dangerously loud sounds to apply tension to the ossicles of the inner ear and thereby reduce the amplitude of vibrations reaching the cochlea. As useful as the determination of an SRT can be for fitting the patient with the cochlear implant, however, current techniques for determining SRTs leave room for improvement. For example, current SRT determination techniques typically require additional instrumentation (e.g., middle ear analyzers, extra electrodes beyond the electrodes used to provide the electrical stimulation, etc.) to be inserted into the ear. Such additional instrumentation may be inconvenient, risky, painful, and otherwise detrimental to the patient and/or the clinicians and other medical personnel working with the patient.